Previous work on the interaction of BrAcFsk, an alkylating derivative of forskolin, with adenylyl cyclase showed that BrAcFsk covalently reacted with nucleophilic groups on adenylyl cyclase. BrAcFsk irreversibly inhibits photoincorporation of an iodinated arylazido derivative of forskolin, 125-I-6-AIPP-Fsk, into adenylyl cyclase. These results indicated that the alkylating derivative and the photoaffinity derivative are binding at the same site. BrAcFsk inhibits photoincorporation of another arylazido derivative of forskolin, 125--I-7-AIPP-Fsk, into the glucose transporter, however the inhibition was reversible. These results suggest that in contrast to the adenylyl cyclase, there is no reactive nucleophilic group at the forskolin binding site on the human erythrocyte glucose transporter. We are studying the interaction of 125-I-6-AIPP-Fck with recombinant adenylyl cyclases expressed in sf9 cells and 293 cells. The type I adenylyl cyclase is labeled in the absence of Gs only slightly increases the labeling efficiency. In contrast, type II adenyl cyclase is not labeled as well as type I adenylyl cyclase in the absence of Gs. The addition of Gs increases the labeling efficiency of type II by almost an order of magnitude. Preliminary results suggest that the photolabel does not label type III adenylyl cyclase.